The receptor tyrosine kinases (RTKs) are transmembrane proteins and function as sensors for extracellular ligands, which transduce signals from extracellular medium to the cytoplasm. Their activation leads to the recruitment, phosphorylation, and activation of the downstream signaling pathways, which ultimately regulate cellular functions such as proliferation, growth, differentiation and motility. Abnormal overexpression levels and/or enhanced activities of RTKs have been associated with a variety of human cancers, leading to a strong interest in the development of inhibitors against these kinases.
Tyro-3, Axl, and Mer constitute the TAM family of RTKs characterized by a conserved sequence within the kinase domain and adhesion molecule-like extracellular domains. With varying degree of specificity and affinity, TAM kinases can be activated by the vitamin K-dependent ligand Gas6 and/or Protein S. Strong evidence supports their association with both cancer (gain-in-function) and autoimmunity (loss-of-function). TAM kinase signaling has been implicated in a myriad of cellular responses, many of which are the hallmarks of cancer, including proliferation, survival, migration, invasion and angiogenesis. In addition, TAM plays pivotal roles innate immunity through inhibiting inflammation in macrophages and dendritic cells and promoting the phagocytosis of apoptotic cells. While the oncogenic activity of TAM kinases appears to be mediated via PI3K/AKT pathway, the JAK-STAT pathway is critical for their roles in immune responses. Overexpression of TAM kinases has been observed in over 20 human cancers. The level of their expression was shown to correlate with shorter progression-free and overall survival and their up-regulation has been linked to cancer resistance to cytotoxic drugs and targeted therapies.
While broadly expressed in various human tumor cell lines, Tyro3, Axl, and Mer exhibit their respective tissue-specific expression patterns. Tyro-3 is highly expressed in the nervous system whereas, Axl is expressed ubiquitously. Higher level of Mer is often found in hematopoietic lineages such as in monocytes/macrophages, dendritic cells, NK cells, NKT cells, megakaryocytes, and platelets.
Compared to Axl and Mer, Tyro3 is the least studied kinase of the TAM family. Implication of Tyro3 in tumorigenesis was only recently substantiated by recent studies, which revealed Tyro3 is a potential oncogene in melanoma that is linked to poorer outcome of patients suffering from melanoma regardless the BRAF or NRAS status by conferring survival advantage to melanoma cells. It was also identified as one of kinases significantly up-regulated in lung cancer by a phosphoproteomic screen. High level of Tyro3 expression has also been correlated with thyroid cancer.
As the founding member of the TAM kinase family, Axl was discovered as a transforming gene in chronic myelogenous leukemia (CML). Axl overexpression has since been reported in a wide range of human malignancies and is associated with invasiveness and metastasis in lung, prostate, breast and pancreatic cancer. Axl is also an important regulator of breast cancer metastasis and EMT. Activation of the Axl kinase confers resistance to EGFR targeted therapy in lung cancer. Upregulation of Axl has been implicated as a mechanism of resistance to imatinib in CML and gastrointestinal stromal tumors and to lapatinib in breast cancer. Axl expression has also been associated with chemoresistance in AML, NSCLC and ovarian cancer.
Mer is overexpressed/overactivated in a wide variety of cancers and has been established as a therapeutic target in hematopoietic malignancies and solid tumors including leukemia, non-small cell lung cancer, glioblastoma, melanoma, prostate cancer, breast cancer, colon cancer, gastric cancer, pituitary adenomas, and rhabdomyosarcomas. Oncogenic potential of Mer is mediated through the activation of several canonical oncogenic signaling pathways including the mitogen-activated protein kinase and phosphoinositide 3-kinase pathways, as well as regulation of signal transducer and activator of transcription family members, migration-associated proteins including the focal adhesion kinase and myosin light chain 2, and prosurvival proteins such as survivin and Bcl-2. In neoplastic cells, these signaling events result in functional phenotypes such as decreased apoptosis, increased migration, chemoresistance, increased colony formation, and increased tumor formation in murine models. Conversely, Mer inhibition by genetic or pharmacologic means can reverse these pro-oncogenic phenotypes.
The following literature reports small molecule inhibitors of Tyro3, Axl and Mer: Zhang et al., J. Med. Chem., 2014, 57, 7031-7041; Rho et al., Cancer Res., 2014, 74, 253-262; Traoré et al., Euro. J. Med. Chem., 2013, 70, 789-801; Zhang et al., J. Med. Chem., 2013, 56, 9683-9692; Zhang et al., J. Med. Chem., 2013, 56, 9693-9700; Liu et al. Euro. J. Med. Chem., 2013, 65, 83-93; Powell et al. Bioorg. Med. Chem. Lett., 2013, 23, 1051-1055; Powell et al. Bioorg. Med. Chem. Lett., 2013, 23, 1046-1050; Suárez et al. Euro. J. Med. Chem., 2013, 61, 2-25; M. F. Burbridge et al. Mol. Cancer Ther., 2013, 12, 1749-1762; Powell et al. Bioorg. Med. Chem. Lett. 2012, 22, 190-193; Liu et al. ACS Med. Chem. Lett., 2012, 3, 129-134; Mollard et al. ACS Med. Chem. Lett., 2011, 2, 907-912; Holland et al. Cancer Res., 2010, 70(4), 1544-1554.; Ono et al. poster number MEDI-393, 244th ACS National Meeting & Exposition, Philadelphia, Pa., Aug. 19-23, 2012; Zhang et al. poster number MEDI-56, 244th ACS National Meeting & Exposition, Philadelphia, Pa., Aug. 19-23, 2012; Yang et al. poster number MEDI-265, 242nd ACS National Meeting & Exposition, Denver, Colo., Aug. 28-Sep. 1, 2011; Zhang et al. poster number MEDI-62, 242nd ACS National Meeting & Exposition, Denver, Colo., Aug. 28-Sep. 1, 2011; Wang et al. poster number MEDI-18, 242nd ACS National Meeting & Exposition, Denver, Colo., Aug. 28-Sep. 1, 2011; Huang et al. J. Stru. Biol. 2009, 165, 88-96. Axl inhibitors have also been disclosed in US2008188455A1; WO2007030680A2; WO2008045978A1; WO2008080134A2; WO2008083353A1; WO2008083354A1; WO2008083356A1; WO2008083357A1; WO2008083367A2; WO2008128072A2; WO2009007390A2; WO2009024825A1; WO2009047514A1; WO2009053737A2; WO2009054864A1; WO2009127417A1; WO2010005876A2; WO2010005879A1; WO2010083465A1; WO2010090764A1; WO2011045084A1; WO2011138751A2; WO2012028332A1; WO2012135800A1; WO2013074633A1; WO2013115280A1 and WO2013162061A1.
Quinalzolines have been reported in the following literature reports: Besson et. al. Tetrahedron, 1998, 54, 6475-6484; Jin et. al. Bioorg. Med. Chem. 2005, 13, 5613-5622; Jung et. al. J. Med. Chem. 2006, 49, 955-970; Hennequin et. al. J. Med. Chem. 2006, 49, 6465-6488; Fray et. al. Tetrahedron Lett. 2006, 47, 6365-6368; Jin et. al. Bioorg. Med. Chem. Lett. 2006, 16, 5864-5869; Yang et al. Bioorg. Med. Chem. Lett. 2007, 17, 2193-2196; Duncton et al. J. Org. Chem., 2009, 74, 6354-6357; Guiles et al. Bioorg. Med. Chem. Lett. 2009, 19, 800-802; Zhang et al. Clin. Cancer Res. 2011, 17, 4439-50; Li et al. J. Med. Chem., 2012, 55, 3852-3866; Plé et al. Bioorg. Med. Chem. Lett. 2012, 22, 262-266; WO2000021955A1; WO2001094341A1; WO2003040109A2; WO2003045395A1; WO2003055491A1; WO2003055866A1; US20040038992A1; WO2004094401A1; WO2006040526A1; WO2006067391A1; WO2006129064A1; WO2007083096A2; WO2007117161A1; WO2009117080A1; WO2010136475A1; CN101747329A; and CN102382065A.